Large area wood chips, so-called wafers or strands, are preferably glued in free fall or gravity gluing mixers, in which a cylindrical mixing drum rotatable about its central longitudinal axis is provided with an inlet aperture for the wood chips in one end wall and an outlet aperture for the glued wood chips in the other end wall. On the inner wall of the mixing drum are provided shovel-like plates by means of which chips are raised from a wood chip bed in the inner area of the mixing drum and are returned in free fall to the latter in the form of a relatively thin layer. Such an apparatus is disclosed, for example, in U.S. Pat. No. 4,188,130. Generally, no gluing problems are encountered when the chips are glued with powdered glue. Various processes exist for gluing chips with liquid glue, which is being ever increasingly used and they in part suffer from serious shortcomings with respect to handling, uniformity of the glue distribution on the chips (gluing quality), dirtying of the mixer and operating costs. Thus, it is known to provide pressure or rotary spray nozzles on the end wall having the inlet aperture. This arrangement leads to a poor gluing quality because only a limited chip surface is sprayed with glue. In addition, the mixer is made very dirty, because all the glue is fed in at the beginning of the mixing drum.
It is also known in this connection to supply liquid glue by means of rotary deflectors, whereof one is arranged on the end wall having the inlet aperture and another on the end wall having the outlet aperture. Although the degree of dirtying of the mixer is satisfactory, the gluing quality varies, which leads to increased glue consumption.
Finally, it is also known to distribute liquid glue in a pressureless manner by means of two-fluid nozzles, i.e. together with compressed air. The gluing quality is good, but the mixer becomes very dirty.